1. Technical Field
The present invention relates to a synchronization point detection method for detecting the timings of a synchronization point and a power line communication apparatus using the same.
2. Background Art
In recent years, a power line communication (PLC) technology that uses a household power line as a transmission channel for communication has been put into practical use, and a power line communication apparatus employing this technology such as a PLC modem has been commercialized,
For instance, when a first power line communication apparatus and a second power line communication apparatus connected to a common power line coexist on the common power line and bidirectional communication is carried out between these apparatuses, only one power line is available as a signal transmission channel. Therefore, it is necessary to synchronize the transmission/reception timings between the first power line communication apparatus and the second power line communication apparatus so that the communication can be carried out by efficiently using the transmission channel while preventing signals transmitted by the first power line communication apparatus from colliding with signal transmitted by the second power line communication apparatus on the transmission channel.
In the power line communication, since a sinusoidal alternating power of AC 100 V or the like is generally supplied through the power line as the transmission channel, a reference time point can be detected from the waveform of the alternating power and used for synchronization of the communication. That is to say, since the common alternating power is applied through the power line to respective PLC modems, the respective PLC modems are able to synchronize the timings of the communication controlled by respective station (PLC modems) by detecting the reference time point from the waveform of the alternating power.
As a method for detecting the synchronization point, the following technologies are known. The technology disclosed in JP-A-9-116525 uses a photo coupler including a photo diode to generate a signal indicating a reception period which is roughly synchronized with the power cycle based on a comparison between the waveform of the power source and a predetermined threshold value. The technology uses a spread spectrum modulation/demodulation method to generate a transmission timing signal at the time of demodulating data received within the roughly synchronized period, thereby taking more exact synchronization.
The technology disclosed in JP-A-10-290184 calculates the average cycle period of an alternating power using the ON/OFF timings of the reception timings of received transmission signals to detect the reception timings of the transmission signals in the subsequent cycle period based on the zero crossing time points of an alternating power and the average cycle period.
The technology disclosed in JP-A-2003-008479 detects the period of time, during which a power cycle period has a specific phase, from a cycle period twice the fundamental cycle period that is synchronized with the power cycle period.
In the technology disclosed in JP-A-9-116525, since the spread spectrum modulation/demodulation method is used, the transmission/reception timings are determined such that the timings on the transmitting side are generated using the timings detected on the receiving side when decoding the received data. However, it is difficult to determine the accurate transmission/reception timings in advance of the transmission and reception.
As a method of determining the timings in advance using a power waveform, JP-A-9-116525 discloses a method of detecting the zero crossing point of an alternating waveform of a power based on a comparison between the power waveform and a predetermined value.
However, there is a possibility that a relative large detection error may occur in the detected timing. For instance, in the case of using a photo coupler, the characteristics of the CTR (Current Transfer Ratio) of the photo coupler may become the cause of the error. The CTR of the photo coupler represents the ratio of the current flowing through a photo transistor on the output side to the current flowing through a light emitting diode on the input side. The CTR of the photo coupler typically has a value around 50 to 300 due to individual variation and may vary further due to aged deterioration or an ambient temperature.
When the CTR of the photo coupler varies, the relationship between the operating point (threshold voltage; 0.7 V, for example) of a transistor connected to the photo coupler itself or to the output side thereof and the alternating waveform voltage of a power varies. As a result, the time point at which the zero crossing point of the alternating waveform is detected may deviate.
If such a large time difference (220 to 250 μsec) occurs when detecting the zero crossing point of the alternating waveform, in the above-described power line communication apparatus, a large error may occur in the communication timings. Therefore, when the allowable time margin is small as in the case of time slots with a small time width, there is a possibility that signals collide with each other on the power line as the common transmission channel, thereby making it difficult to achieve high-speed communication.
However, when a circuit element such as a photo coupler exhibiting a small variation in the CTR characteristics, for example, is selected, or a circuit element capable of compensating for the influence of a secular change or environmental change is added, the manufacturing cost may increase greatly.
Although the technologies disclosed in JP-A-9-116525, JP-A-10-290184 and JP-A-2003-008479 can synchronize the PLC modem to the power cycle without being influenced by the variation, it is difficult with the technologies for the respective PLC modems to detect a reference time point from the waveform of the alternating power, thereby accurately synchronizing the timings of the communication controlled by the respective stations (PLC modems).
Although the technologies disclosed in JP-A-10-290184 and JP-A-2003-008479 can detect the accurate power cycle, it is difficult with the technologies to detect the accurate zero crossing point which becomes the basis of the power cycle.